Question

(a) Our skin is damaged by UV rays from the sun. Explain how our skin sells prevent mutation from accumulating.

(b) Explain how telomeres are related to ageing, and cancer progression.

Answer 1

A. UV rays are non ionizing mutagenic agents. They cause spontaneous mutations which arise during DNA replication.

UV rays cause the formation of pyrimidine dimers specially thymine dimers. They result in the formation of covalent bond between adjacent thymine dimers on the same strand. This lesion is repaired in the presence of visible light by enzyme photolyase. This is called as photoreactivation. There is no excision of bases, the mutated thymine dimers are repaired by the enzyme so, that they are not passed on to the daughter cells.

B. The linear chromosomes of eukaryotes get shortened after every round of DNA replication. So, after multiple rounds of DNA replication the length of the chromosome drastically decreases and the cell is made to die by apoptosis. This is required because loss of DNA from the cell results in the loss of genes from the chromosomes due to deletion mutation. This results in the aging of the cell. Apoptosis is programmed cell death by which damaged cell is made to die so, that it no longer participates in cell division and gives rise to mutant daughter cells.

So, there is an enzyme called telomerase which adds repeatitive sequences called telomeres at the tips of the chromosomes to restore their length. This enzyme is a reverse transcriptase and provides an RNA template for the synthesis of DNA.

In cancerous cells, the expression of enzyme telomerase is increased and this is the reason they are able to divide uncontrollably.

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